After I realized that Unity is overstretching my graphics card and my CPU, I decided to remain to Ubuntu by switching to Xubuntu. Xubuntu is must faster, but I am now trying to reproduce the handy window handling behaviour with keyboard shortcuts, as it used to be done by Compiz in Unity.

xubuntu 12.10 window tiling shortcuts explains how to enable this functionality for side-by-side or top-to-bottom tiling. But in order to fit 4 windows on the screen, I still need a shortcut for moving a window on the screen corner, like Alt+Ctrl+Num used to be for Unity.